Is there a way to access an iteration-counter in Java s for-each loop

Question

Is there a way in Java s for-each loop  for String s   stringArray      doSomethingWith s       to find out how often the loop has already been processed   Aside from using the old and well-known for int i 0  i  lt  boundary  i    - loop  is the construct  int i   0  for String s   stringArray      doSomethingWith s     i        the only way to have such a counter available in a for-each loop

User · Answer

The easiest solution is to just run your own counter thus:

int i = 0;
for (String s : stringArray) {
    doSomethingWith(s, i);
    i++;
}

The reason for this is because there's no actual guarantee that items in a collection (which that variant of for iterates over) even have an index, or even have a defined order (some collections may change the order when you add or remove elements).

See for example, the following code:

import java.util.*;

public class TestApp {
  public static void AddAndDump(AbstractSet<String> set, String str) {
    System.out.println("Adding [" + str + "]");
    set.add(str);
    int i = 0;
    for(String s : set) {
        System.out.println("   " + i + ": " + s);
        i++;
    }
  }

  public static void main(String[] args) {
    AbstractSet<String> coll = new HashSet<String>();
    AddAndDump(coll, "Hello");
    AddAndDump(coll, "My");
    AddAndDump(coll, "Name");
    AddAndDump(coll, "Is");
    AddAndDump(coll, "Pax");
  }
}

When you run that, you can see something like:

Adding [Hello]
   0: Hello
Adding [My]
   0: Hello
   1: My
Adding [Name]
   0: Hello
   1: My
   2: Name
Adding [Is]
   0: Hello
   1: Is
   2: My
   3: Name
Adding [Pax]
   0: Hello
   1: Pax
   2: Is
   3: My
   4: Name

indicating that, rightly so, order is not considered a salient feature of a set.

There are other ways to do it without a manual counter but it's a fair bit of work for dubious benefit.

User · Answer

There is a  variant  to pax  answer      -   int i   -1  for String s   stringArray        doSomethingWith s    i

User · Answer

One of the changes Sun is considering for Java7 is to provide access to the inner Iterator in foreach loops  the syntax will be something like this  if this is accepted    for  String str   list   it      if  str length    gt  100        it remove            This is syntactic sugar  but apparently a lot of requests were made for this feature  But until it is approved  you ll have to count the iterations yourself  or use a regular for loop with an Iterator

User · Answer

The easiest solution is to just run your own counter thus:

int i = 0;
for (String s : stringArray) {
    doSomethingWith(s, i);
    i++;
}

The reason for this is because there's no actual guarantee that items in a collection (which that variant of for iterates over) even have an index, or even have a defined order (some collections may change the order when you add or remove elements).

See for example, the following code:

import java.util.*;

public class TestApp {
  public static void AddAndDump(AbstractSet<String> set, String str) {
    System.out.println("Adding [" + str + "]");
    set.add(str);
    int i = 0;
    for(String s : set) {
        System.out.println("   " + i + ": " + s);
        i++;
    }
  }

  public static void main(String[] args) {
    AbstractSet<String> coll = new HashSet<String>();
    AddAndDump(coll, "Hello");
    AddAndDump(coll, "My");
    AddAndDump(coll, "Name");
    AddAndDump(coll, "Is");
    AddAndDump(coll, "Pax");
  }
}

When you run that, you can see something like:

Adding [Hello]
   0: Hello
Adding [My]
   0: Hello
   1: My
Adding [Name]
   0: Hello
   1: My
   2: Name
Adding [Is]
   0: Hello
   1: Is
   2: My
   3: Name
Adding [Pax]
   0: Hello
   1: Pax
   2: Is
   3: My
   4: Name

indicating that, rightly so, order is not considered a salient feature of a set.

There are other ways to do it without a manual counter but it's a fair bit of work for dubious benefit.

User · Answer

Idiomatic Solution  final Set lt Double gt  doubles     boilerplate final Iterator lt Double gt  iterator   doubles iterator    for  int ordinal   0  iterator hasNext    ordinal          System out printf  quot  d  f quot  ordinal iterator next         System out println        this is actually the solution that Google suggests in the Guava discussion on why they did not provide a CountingIterator

User · Answer

There is another way   Given that you write your own Index class and a static method that returns an Iterable over instances of this class you can   for  Index lt String gt  each  With index stringArray         each value      each index              Where the implementation of With index is something like  class With       public static  lt T gt  Iterable lt Index lt T gt  gt  index final T   array            return new Iterable lt Index lt T gt  gt                  public Iterator lt Index lt T gt  gt  iterator                     return new Iterator lt Index lt T gt  gt                          index   0                      public boolean hasNext     return index  lt  array size                       public Index lt T gt  next     return new Index array index   index

User · Answer

No  but you can provide your own counter   The reason for this is that the for-each loop internally does not have a counter  it is based on the Iterable interface  i e  it uses an Iterator to loop through the  collection  - which may not be a collection at all  and may in fact be something not at all based on indexes  such as a linked list

User · Answer

If you need a counter in an for-each loop you have to count yourself  There is no built in counter as far as I know

User · Answer

I m afraid this isn t possible with foreach  But I can suggest you a simple old-styled for-loops       List lt String gt  l   new ArrayList lt String gt          l add  a        l add  b        l add  c        l add  d            the array     String   array   new String l size          for ListIterator lt String gt  it  l listIterator    it hasNext                    array it nextIndex      it next            Notice that  the List interface gives you access to it nextIndex      edit   To your changed example       for ListIterator lt String gt  it  l listIterator    it hasNext                    int i   it nextIndex            doSomethingWith it next    i

User · Answer

If you need a counter in an for-each loop you have to count yourself  There is no built in counter as far as I know

User · Answer

There is another way   Given that you write your own Index class and a static method that returns an Iterable over instances of this class you can   for  Index lt String gt  each  With index stringArray         each value      each index              Where the implementation of With index is something like  class With       public static  lt T gt  Iterable lt Index lt T gt  gt  index final T   array            return new Iterable lt Index lt T gt  gt                  public Iterator lt Index lt T gt  gt  iterator                     return new Iterator lt Index lt T gt  gt                          index   0                      public boolean hasNext     return index  lt  array size                       public Index lt T gt  next     return new Index array index   index

User · Answer

The easiest solution is to just run your own counter thus:

int i = 0;
for (String s : stringArray) {
    doSomethingWith(s, i);
    i++;
}

The reason for this is because there's no actual guarantee that items in a collection (which that variant of for iterates over) even have an index, or even have a defined order (some collections may change the order when you add or remove elements).

See for example, the following code:

import java.util.*;

public class TestApp {
  public static void AddAndDump(AbstractSet<String> set, String str) {
    System.out.println("Adding [" + str + "]");
    set.add(str);
    int i = 0;
    for(String s : set) {
        System.out.println("   " + i + ": " + s);
        i++;
    }
  }

  public static void main(String[] args) {
    AbstractSet<String> coll = new HashSet<String>();
    AddAndDump(coll, "Hello");
    AddAndDump(coll, "My");
    AddAndDump(coll, "Name");
    AddAndDump(coll, "Is");
    AddAndDump(coll, "Pax");
  }
}

When you run that, you can see something like:

Adding [Hello]
   0: Hello
Adding [My]
   0: Hello
   1: My
Adding [Name]
   0: Hello
   1: My
   2: Name
Adding [Is]
   0: Hello
   1: Is
   2: My
   3: Name
Adding [Pax]
   0: Hello
   1: Pax
   2: Is
   3: My
   4: Name

indicating that, rightly so, order is not considered a salient feature of a set.

There are other ways to do it without a manual counter but it's a fair bit of work for dubious benefit.

User · Answer

For situations where I only need the index occasionally  like in a catch clause  I will sometimes use indexOf   for String s   stringArray      try       doSomethingWith s       catch  Exception e        LOGGER warn  Had some kind of problem with string           stringArray indexOf s           s  e

User · Answer

One of the changes Sun is considering for Java7 is to provide access to the inner Iterator in foreach loops  the syntax will be something like this  if this is accepted    for  String str   list   it      if  str length    gt  100        it remove            This is syntactic sugar  but apparently a lot of requests were made for this feature  But until it is approved  you ll have to count the iterations yourself  or use a regular for loop with an Iterator

User · Answer

No  but you can provide your own counter   The reason for this is that the for-each loop internally does not have a counter  it is based on the Iterable interface  i e  it uses an Iterator to loop through the  collection  - which may not be a collection at all  and may in fact be something not at all based on indexes  such as a linked list

User · Answer

Using lambdas and functional interfaces in Java 8 makes creating new loop abstractions possible  I can loop over a collection with the index and the collection size   List lt String gt  strings   Arrays asList  one    two   three   four    forEach strings   x  i  n  - gt  System out println       i 1        n        x      Which outputs   1 4  one 2 4  two 3 4  three 4 4  four   Which I implemented as       FunctionalInterface    public interface LoopWithIndexAndSizeConsumer lt T gt           void accept T t  int i  int n           public static  lt T gt  void forEach Collection lt T gt  collection                                    LoopWithIndexAndSizeConsumer lt T gt  consumer          int index   0        for  T object   collection            consumer accept object  index    collection size                    The possibilities are endless  For example  I create an abstraction that uses a special function just for the first element   forEachHeadTail strings                    head  - gt  System out print head                     tail  - gt  System out print     tail      Which prints a comma separated list correctly   one two three four   Which I implemented as   public static  lt T gt  void forEachHeadTail Collection lt T gt  collection                                          Consumer lt T gt  headFunc                                          Consumer lt T gt  tailFunc       int index   0     for  T object   collection         if  index      0            headFunc accept object                 else           tailFunc accept object                    Libraries will begin to pop up to do these sorts of things  or you can roll your own

User · Answer

I m a little surprised no-one suggested the following  I admit it s a lazy approach      If stringArray is a List of some sort  you could use something like stringArray indexOf S  to return a value for the current count   Note  this assumes that the elements of the List are unique  or that it doesn t matter if they are non-unique  because in that case it will return the index of the first copy found    There are situations in which that would be sufficient

User · Answer

One of the changes Sun is considering for Java7 is to provide access to the inner Iterator in foreach loops  the syntax will be something like this  if this is accepted    for  String str   list   it      if  str length    gt  100        it remove            This is syntactic sugar  but apparently a lot of requests were made for this feature  But until it is approved  you ll have to count the iterations yourself  or use a regular for loop with an Iterator

User · Answer

If you need a counter in an for-each loop you have to count yourself  There is no built in counter as far as I know

User · Answer

Idiomatic Solution  final Set lt Double gt  doubles     boilerplate final Iterator lt Double gt  iterator   doubles iterator    for  int ordinal   0  iterator hasNext    ordinal          System out printf  quot  d  f quot  ordinal iterator next         System out println        this is actually the solution that Google suggests in the Guava discussion on why they did not provide a CountingIterator

User · Answer

The best and optimized solution is to do the following thing   int i 0   for Type t  types               i        Where Type can be any data type and types is the variable on which you are applying for a loop

User · Answer

Though there are soo many other ways mentioned to achieve the same  I will share my way for some unsatisfied users  I am using the Java 8 IntStream feature   1  Arrays    Object   obj    1 2 3 4 5 6 7   IntStream range 0  obj length  forEach index- gt        System out println  index      index       System out println  value      obj index          2  List    List lt String gt  strings   new ArrayList lt String gt     Collections addAll strings  A   B   C   D     IntStream range 0  strings size    forEach index- gt        System out println  index      index       System out println  value      strings get index

User · Answer

I m afraid this isn t possible with foreach  But I can suggest you a simple old-styled for-loops       List lt String gt  l   new ArrayList lt String gt          l add  a        l add  b        l add  c        l add  d            the array     String   array   new String l size          for ListIterator lt String gt  it  l listIterator    it hasNext                    array it nextIndex      it next            Notice that  the List interface gives you access to it nextIndex      edit   To your changed example       for ListIterator lt String gt  it  l listIterator    it hasNext                    int i   it nextIndex            doSomethingWith it next    i

User · Answer

Java 8 introduced the Iterable forEach     Map forEach   method  which is more efficient for many Collection   Map implementations compared to the  classical  for-each loop  However  also in this case an index is not provided  The trick here is to use AtomicInteger outside the lambda expression  Note  variables used within the lambda expression must be effectively final  that is why we cannot use an ordinary int   final AtomicInteger indexHolder   new AtomicInteger    map forEach  k  v  - gt        final int index   indexHolder getAndIncrement           use the index

User · Answer

Here is an example of how I did this  This gets the index at the for each loop  Hope this helps    public class CheckForEachLoop        public static void main String   args             String   months   new String      JANUARY    FEBRUARY    MARCH    APRIL    MAY    JUNE    JULY    AUGUST                    SEPTEMBER    OCTOBER    NOVEMBER    DECEMBER             for  String s   months                if  s    months 2        location where you can change               doSomethingWith s      however many times s and months                                      doSomethingWith s  will be completed and                                       added together instead of counter                                  System out println s

User · Answer

No  but you can provide your own counter   The reason for this is that the for-each loop internally does not have a counter  it is based on the Iterable interface  i e  it uses an Iterator to loop through the  collection  - which may not be a collection at all  and may in fact be something not at all based on indexes  such as a linked list

User · Answer

No  but you can provide your own counter   The reason for this is that the for-each loop internally does not have a counter  it is based on the Iterable interface  i e  it uses an Iterator to loop through the  collection  - which may not be a collection at all  and may in fact be something not at all based on indexes  such as a linked list

User · Answer

If you need a counter in an for-each loop you have to count yourself  There is no built in counter as far as I know

User · Answer

There is a  variant  to pax  answer      -   int i   -1  for String s   stringArray        doSomethingWith s    i

User · Answer

Here is an example of how I did this  This gets the index at the for each loop  Hope this helps    public class CheckForEachLoop        public static void main String   args             String   months   new String      JANUARY    FEBRUARY    MARCH    APRIL    MAY    JUNE    JULY    AUGUST                    SEPTEMBER    OCTOBER    NOVEMBER    DECEMBER             for  String s   months                if  s    months 2        location where you can change               doSomethingWith s      however many times s and months                                      doSomethingWith s  will be completed and                                       added together instead of counter                                  System out println s

User · Answer

The best and optimized solution is to do the following thing   int i 0   for Type t  types               i        Where Type can be any data type and types is the variable on which you are applying for a loop

User · Answer

The easiest solution is to just run your own counter thus:

int i = 0;
for (String s : stringArray) {
    doSomethingWith(s, i);
    i++;
}

The reason for this is because there's no actual guarantee that items in a collection (which that variant of for iterates over) even have an index, or even have a defined order (some collections may change the order when you add or remove elements).

See for example, the following code:

import java.util.*;

public class TestApp {
  public static void AddAndDump(AbstractSet<String> set, String str) {
    System.out.println("Adding [" + str + "]");
    set.add(str);
    int i = 0;
    for(String s : set) {
        System.out.println("   " + i + ": " + s);
        i++;
    }
  }

  public static void main(String[] args) {
    AbstractSet<String> coll = new HashSet<String>();
    AddAndDump(coll, "Hello");
    AddAndDump(coll, "My");
    AddAndDump(coll, "Name");
    AddAndDump(coll, "Is");
    AddAndDump(coll, "Pax");
  }
}

When you run that, you can see something like:

Adding [Hello]
   0: Hello
Adding [My]
   0: Hello
   1: My
Adding [Name]
   0: Hello
   1: My
   2: Name
Adding [Is]
   0: Hello
   1: Is
   2: My
   3: Name
Adding [Pax]
   0: Hello
   1: Pax
   2: Is
   3: My
   4: Name

indicating that, rightly so, order is not considered a salient feature of a set.

There are other ways to do it without a manual counter but it's a fair bit of work for dubious benefit.

User · Answer

Though there are soo many other ways mentioned to achieve the same  I will share my way for some unsatisfied users  I am using the Java 8 IntStream feature   1  Arrays    Object   obj    1 2 3 4 5 6 7   IntStream range 0  obj length  forEach index- gt        System out println  index      index       System out println  value      obj index          2  List    List lt String gt  strings   new ArrayList lt String gt     Collections addAll strings  A   B   C   D     IntStream range 0  strings size    forEach index- gt        System out println  index      index       System out println  value      strings get index

User · Answer

I m a little surprised no-one suggested the following  I admit it s a lazy approach      If stringArray is a List of some sort  you could use something like stringArray indexOf S  to return a value for the current count   Note  this assumes that the elements of the List are unique  or that it doesn t matter if they are non-unique  because in that case it will return the index of the first copy found    There are situations in which that would be sufficient

User · Answer

I m afraid this isn t possible with foreach  But I can suggest you a simple old-styled for-loops       List lt String gt  l   new ArrayList lt String gt          l add  a        l add  b        l add  c        l add  d            the array     String   array   new String l size          for ListIterator lt String gt  it  l listIterator    it hasNext                    array it nextIndex      it next            Notice that  the List interface gives you access to it nextIndex      edit   To your changed example       for ListIterator lt String gt  it  l listIterator    it hasNext                    int i   it nextIndex            doSomethingWith it next    i

User · Answer

Using lambdas and functional interfaces in Java 8 makes creating new loop abstractions possible  I can loop over a collection with the index and the collection size   List lt String gt  strings   Arrays asList  one    two   three   four    forEach strings   x  i  n  - gt  System out println       i 1        n        x      Which outputs   1 4  one 2 4  two 3 4  three 4 4  four   Which I implemented as       FunctionalInterface    public interface LoopWithIndexAndSizeConsumer lt T gt           void accept T t  int i  int n           public static  lt T gt  void forEach Collection lt T gt  collection                                    LoopWithIndexAndSizeConsumer lt T gt  consumer          int index   0        for  T object   collection            consumer accept object  index    collection size                    The possibilities are endless  For example  I create an abstraction that uses a special function just for the first element   forEachHeadTail strings                    head  - gt  System out print head                     tail  - gt  System out print     tail      Which prints a comma separated list correctly   one two three four   Which I implemented as   public static  lt T gt  void forEachHeadTail Collection lt T gt  collection                                          Consumer lt T gt  headFunc                                          Consumer lt T gt  tailFunc       int index   0     for  T object   collection         if  index      0            headFunc accept object                 else           tailFunc accept object                    Libraries will begin to pop up to do these sorts of things  or you can roll your own

User · Answer

I m afraid this isn t possible with foreach  But I can suggest you a simple old-styled for-loops       List lt String gt  l   new ArrayList lt String gt          l add  a        l add  b        l add  c        l add  d            the array     String   array   new String l size          for ListIterator lt String gt  it  l listIterator    it hasNext                    array it nextIndex      it next            Notice that  the List interface gives you access to it nextIndex      edit   To your changed example       for ListIterator lt String gt  it  l listIterator    it hasNext                    int i   it nextIndex            doSomethingWith it next    i

User · Answer

For situations where I only need the index occasionally  like in a catch clause  I will sometimes use indexOf   for String s   stringArray      try       doSomethingWith s       catch  Exception e        LOGGER warn  Had some kind of problem with string           stringArray indexOf s           s  e

User · Answer

One of the changes Sun is considering for Java7 is to provide access to the inner Iterator in foreach loops  the syntax will be something like this  if this is accepted    for  String str   list   it      if  str length    gt  100        it remove            This is syntactic sugar  but apparently a lot of requests were made for this feature  But until it is approved  you ll have to count the iterations yourself  or use a regular for loop with an Iterator

User · Answer

Java 8 introduced the Iterable forEach     Map forEach   method  which is more efficient for many Collection   Map implementations compared to the  classical  for-each loop  However  also in this case an index is not provided  The trick here is to use AtomicInteger outside the lambda expression  Note  variables used within the lambda expression must be effectively final  that is why we cannot use an ordinary int   final AtomicInteger indexHolder   new AtomicInteger    map forEach  k  v  - gt        final int index   indexHolder getAndIncrement           use the index

[java] Is there a way to access an iteration-counter in Java's for-each loop?

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